This invention relates to the process of removing films from substrate materials with reactive gases. More particularly, this invention is concerned with etching substrates (e.g., silicon or polysilicon) to remove from the surface thereof layers or films (e.g., oxides or nitrides of silicon and polysilicons) by action of anhydrous reactive gases, for example anhydrous hydrogen fluoride. The process according to this invention is highly important in the processing of silicon wafers used in the manufacture of semi-conductor integrated circuit chips as well as in other industrial etching and cleaning processes.
Currently, the etching process in the manufacture of semiconductor integrated circuit chips and thin film circuitry has been conducted in a wet acid etching procedure or with plasma techniques. One method of wet etching has been to overlay the surface to be etched with a suitable photoresist mask and to immerse the circuit so masked in a chemical solution which attacks the surface to be etched while leaving the mask otherwise intact. Alternatively, the acid is sprayed onto the surface to be etched. With these known wet etching procedures, etching proceeds at rates at the approximate range of 50 to 2000 angstroms per minute, and is controllable in that range. It has been difficult with the chemical etching processes presently known to achieve well-defined edges on the etched surfaces. The difficulty arises because the chemical action of the wet etchant tends to etch isotropically, i.e., it undercuts the mask by the same distance that it penetrates the underlayer, and thus reduces the final feature dimensions. It is therefore, very difficult to use wet chemical etching to achieve fine structures.
An additional problem in wet etching is that at completion of the etching process, thorough rinsing of the wafers is required to remove residual etchant and other contaminants. Accordingly, after the wafers have been etched, they must be cleaned in a separate step, and cleaning contributes to a particulate problem, especially as relates to heavy metals which may be collected on the surface of the wafer. The steps involved in cleaning wafers after etching involve the use of chemicals such as hydrogen chloride and hydrogen peroxide, or ammonium hydroxide and hydrogen peroxide. These cleaning agents must be rinsed from the surface of the wafers subsequent to the cleaning.
In addition, the wet etching of wafers presents other problems which are difficult to deal with. For instance, liquid etchant and rinse media present serious environmental disposal considerations. In most instances these etchant and rinse media must be collected and disposed of in accordance with strict governmental regulations.
In an effort to alleviate these and other problems associated with previous wet liquid etching techniques, various researchers have endeavored to develop gas phase process procedures utilizing a gaseous etchant medium. Known gas phase methods for etching wafers are plasma etching or reactive ion etching (RIE), wherein a vacuum container is filled with a low pressure gas. A surface or substrate to be etched is covered by a photoresist mask and inserted into the container along with the reactive etchant gas. To perform the etching procedure, a voltage is applied to excite the gas, thereby dissociating it and forming various positive and negative ions, reactive neutral species and electrons. The dissociated species interact with the surface to be etched producing various gaseous reaction products.
One significant problem encountered in plasma etching of silicon oxide is that by using the plasma procedures, the underlying layer on the wafer is etched. As a result, when etching of the oxide film has been completed, etching of the underlying layers in the substrate will continue and produce a roughened, damaged surface due to the additional etching.
The ability to etch thin lines and space widths (3 microns or less) in silicon dioxide over silicon is an important aspect of the plasma or RIE processes used in microcircuit manufacture. However, certain definite drawbacks exist in plasma or RIE techniques. The various plasma generating compositions used as the etchant medium have not yielded consistently satisfactory results. Use of the plasma etching technique necessitates various expensive facilities and controls, for providing and regulating the high voltage required, and the vacuum and temperature conditions that are essential. These disadvantages pertaining to plasma etching have led researchers to investigate economically attractive alternatives using gaseous etchants under various conditions.
Certain Russian writers, V. V. Tyapkina and N. S. Guseva reported in an article INTERACTION OF A SILICON SURFACE WITH FLUORINE AND HYDROGEN FLUORIDE, published in the May 1966 issue of RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY, investigating a dry etching procedure utilizing anhydrous hydrogen fluoride with a carrier gas, involving pretreatment of the surface to be etched with gaseous fluorine, prior to introduction of the hydrogen fluoride gas at about 240.degree. C. These Russian researchers investigated the reaction mechanism involved, suggesting that four reaction steps occur when hydrogen fluoride reacts with silicon oxide and silicon dioxide, ultimately yielding silicon hexafluoride and water. However, the Russian workers were unable to achieve a satisfactory removal rate or control. Their process is not acceptable for the fine precision and reproducibility necessary in the production of semi-conductor circuitry.
In a later article, Russian writers V. A. Arslambekov, K. M. Gorbunova, and N. S. Guseva reported in Izv. Akad. Nauk SSR, Neorgan. Mat., Volume 12, No. 12, pp. 2124-2126, December 1976, certain work related to the reaction of silicon with gaseous hydrogen fluoride. At pressures of 100 torr and temperatures from 180.degree. C. to 400.degree. C., removal rates were exceedingly slow and quite unsatisfactory.
Another gas phase process for etching silicon dioxide (the DryOx process) was reported by R. Bersin and R. Reichelderfer, Solid State Technol., 1977, 20(4), 78-80; and also reported in U.S. Pat. No. 4,127,437. They attempted to use anhydrous hydrogen fluoride in a dry process for etching silicon dioxide without damaging the underlaying silicon substrate. Their conclusion was that both heating and the use of a vacuum reaction chamber were necessary in controlling the reaction. However, this process still lacked the removal rate that is compatible for manufacturing necessary uniformity of removal rate and control.
Japan Kokai Tokyo Koho No. 79,125,143, Sept. 28, 1979, disclosed a hydrogen fluoride gas etching apparatus, wherein the etching chamber is evacuated, the material to be etched is cooled to 20.degree. C. or less, and the etching chamber is then filled with HF to a pressure of 2.0 torr. The process specifically requires that the piece to be etched is mounted on an aluminum plate which is fixed on a thermoelement. Thermocouples are connected to the aluminum supporting plate to control electric current fed to the thermoelement. The purpose is to refrigerate and lower the temperature of the wafer.
Japan Kokai Tokyo Koho No. 79,125,144, Sept. 28, 1979, discloses a related hydrogen fluoride gas etching apparatus, wherein the etching chamber is evacuated, the material to be etched is cooled to between 0.degree. C. to -30.degree. C., and the etching chamber is filled with cold (-30.degree. C.) HF under 5.0 torr. As in the related process described in the paragraph immediately above, this process specifically requires that the material to be etched is mounted on an aluminum supporting plate which is fixed on a thermoelement. Thermocouples are connected to aluminum supporting plate to control electric current fed to the thermoelement.
These previous halogen-containing gas phase etching processes all require specific controls of electric voltage, current, pressure and temperature, and still do not achieve satisfactory control and uniformity of removal rate necessary in the production of semi-conductor materials.